1. Field of the Invention
This invention relates to solenoids and methods for making the same and particularly proportional type solenoids.
2. Description of the Prior Art
General purpose solenoids provide a force-stroke curve whereby the force at a closed stroke gap is higher than the force developed at the initial starting stroke gap. These solenoids are sometimes referred to as "on-off" solenoids and are energized ("on") to a fully operated position or are de-energized ("off") to a fully neutral position. In this type of solenoid, in order to activate the armature to close the stroke gap, the solenoid must only provide enough force to overcome the load including any frictional or side-loading magnetic forces perpendicular to the axis of motion.
Proportional solenoids have long been known in the art to provide a force vs. stroke curve that allows the output force of the solenoid to be proportional to the electrical current applied to the coil. This proportionality of the output force permits such a solenoid to either fully or partially operate a load by selectively applying either the full or a partial electrical current to the solenoid coil and thereby may selectively position the armature along the linear distance of the gap.
In order to operate this type of solenoid accurately, the forces in the solenoid must be accurately controlled. Since the frictional and side loading forces vary depending upon a number of factors that cannot be accurately controlled, including tolerances in manufacturing and the equipment being operated by the solenoid, desirably their effects should be minimized in the design of the solenoid.
The prior art history of proportional solenoids and problems of such solenoids are described in U.S. Pat. No. 3,900,822, Column 1 (Hardwick).
The prior art proportional solenoid provided multiple complex bearing surfaces including a bearing between the armature rod and the stationary pole piece. For example, see the complex bearing and structural support for the armature in each of the prior art patents, German Pat. No. 1,270,178 and U.S. Pat. Nos. 3,870,931 and 3,970,981, in order to provide the necessary structure for a proportional solenoid and to provide concentricity of the armature. Such constructions required very fine manufacturing tolerances, and it was difficult assembling such solenoids.
In order to overcome the concentricity problems of the above prior art patents and provide a concentricity tube for maintaining concentricity of both the armature and fixed pole piece, a multiple section armature tube 10 as shown in FIG. 1 of the drawings was invented. This multiple section tube 10 included a magnetic section 12 made of ferromagnetic material having an external frusto-conical surface 14. The next section of the tube is a non-magnetic brass ring 16 brazed or otherwise permanently fixed at the surface 14 to section 12 and is brazed or permanently fixed along an opposite frusto-conical surface 18 to a third section 20 made of ferromagnetic material. Thus, the non-magnetic brass ring middle section 16 provides the essential non-magnetic radial transverse frusto-conical gap, which gap is linearly coextensive with the stroke gap of the armature. The tube 10 is press fitted or otherwise permanently fixed to a stationary or fixed magnetic pole piece 22 made of ferromagnetic material. The composite armature tube 10 and stationary pole piece 22 are received and mounted in a solenoid coil (not shown).
A movable armature 24 made of ferromagnetic material is provided with a pair of spaced non-magnetic bearing surfaces 26 made by bronze bushings, for example. There is a non-magnetic shim 28 surrounding a push rod 30 permanently mounted on armature 24 and slidable in a center hole 32 of the stationary pole piece 22.
The construction of the three-section tube shown in FIG. 1 is similar to the construction shown in U.S. Pat. No. 3,970,981 except that all three sections are brazed or otherwise fixed together to form one continuous multiple section, multiple metal armature tube.